When fabricating integrated circuits or binary optic devices using contact photolithography techniques, the conventional process used requires the repeated alignment of masks to make contact with wafers, or substrates, in such a manner as to place the mask in registration with one or more features generated in previous steps of the process.
Such contact photolithography process can achieve submicron resolution and has lower capital costs than projection (i.e., non-contact) lithography techniques. Ultimate resolution requires near-perfect contact between the mask and a photoresist-coated substrate, on a sub-wavelength scale. Practically, this near-perfect contact is easier to achieve if either the mask or the substrate can flex and so conform to the opposite member. This flexibility helps correct for imperfect flatness of the contacting surfaces and minimizes the effect of dust particles, which can occur even in the cleanest of clean rooms.
While traditionally masks used in contact lithography are between 1.5 and 3-mm thick, for example, and are quite rigid, there are other reasons for using thinner and more flexible masks when preparing integrated circuits or binary optics devices. First, many substrates are also rigid and it is extremely difficult to assure conformity of contact between two rigid surfaces especially over areas more than a few mm. in diameter. Second, it may be preferable to use copies of the rigid master, e.g., to preserve the master from damage, and whatever the reason for copying a rigid master, it should be copied onto a flexible (i.e., a conformable) mask to preserve fidelity.
Thus, flexible masks allow intimate contact between the entire surface of the mask and the substrate, as is needed for a precise transfer of the mask features. Flexible masks which are sufficiently thin and flexible that they can be brought into precise conformity with the surface of the substrate must have the substrate surrounded by a surface which is at the same height as the surface of the substrate so that the mask does not become distorted because of surface discontinuities at the edges of the substrate. In currently used systems, the substrate is normally held from the underneath by a vacuum chuck arranged in a jig which also supports or includes the surrounding surface. Since such systems are normally designed for use with rigid masks rather than flexible masks, there is no provision for eliminating surface discontinuities at the edges of the substrate. While the surrounding surface may be machined to eliminate discontinuities for a particular substrate, normal tolerances in substrate thickness will result in discontinuities for other substrates, resulting in mask distortion if such systems are used with flexible masks. When using a flexible mask, the combination of the substrate and the surrounding surface would be brought into contact with the flexible mask, and the required vacuum could be drawn to conform the mask to the combination of surfaces. However, this operation would result in distortion of the flexible mask if the surrounding surface is not matched to the substrate.
One exemplary system available for such purpose is the Mask Aligner MA6 manufactured and sold by Karl Suss America, Inc. of Waterbury Center, Vt. Such device is designed normally for use with rigid masks and includes a substrate wedge error corrector (SWEC), as is already known to those in the art.
When attempting to use such device with a flexible mask to check alignment, and to attempt good contact during exposure, a vacuum is pulled between the flexible mask and the substrate. A mismatch in height between the substrate surface and the surrounding surface must be extremely small, e.g., less than about 0.001 inches, to prevent significant distortions in the flexible mask, when a vacuum is pulled, which distortion can result from the mask's contorting as it bridges any discontinuity between the substrate and the surrounding surface. Because the substrate in such device is supported from the back, the combination of wedge angle and thickness tolerances in typical substrates makes it very difficult to ensure that the surrounding surface has the same height as the substrate, within the required tolerance, using the device with flexible or rigid masks.
Such problem in the past has been overcome when using flexible masks by providing an assortment of differently dimensioned jigs and packing shims, each mask and substrate combination requiring a selection of one of a relatively large number of different jigs and the use of a variety of different shims in an attempt to match the substrate and surrounding surfaces appropriately. Accordingly, obtaining a precise matching of the surface heights is tedious, time-consuming, and not always successful.
For example, for a 1-inch diameter substrate having a wedge angle of 30 arc minutes and a thickness tolerance of +/-0.005", the wedge effect alone produces a thickness variation of about 0.009". Even if the substrate surface were precisely level with the surrounding surface at one side and projecting only 0.001" off a precise level at the other side, the cocking that results would cause the substrate edge on the high side to hit and distort a flexible mask when attempting a wedge error correction operation to bring the mask and substrate surfaces parallel.
Accordingly, it is desirable to provide a system, especially useful with flexible masks, while still useful with rigid masks, which precisely aligns a substrate surface and a surrounding surface without the need to provide a variety of differently dimensioned jigs or a plurality of different packing shims.